The monitoring of virtual machines has many applica- tions in areas such as security and systems management. A monitoring technique known as introspection has received significant discussion in the research literature, but these prior works have focused on the applications of introspec- tion rather than how to properly build a monitoring archi- tecture. In this paper we propose a set of requirements that should guide the development of virtual machine monitor- ing solutions. To illustrate the viability of these require- ments, we describe the design of XenAccess, a monitoring library for operating systems running on Xen. XenAccess incorporates virtual memory introspection and virtual disk monitoring capabilities, allowing monitor applications to safely and efficiently access the memory state and disk ac- tivity of a target operating system. XenAccess' efficiency and functionality are illustrated through a series of perfor- mance tests and practical examples.